1. Field of the Invention
The present invention relates to a display device for displaying dot information and more particularly to a display device having a scrolling function.
2. Description of the Prior Art
Conventionally, in CRT display devices of the dot refresh type, a character pattern (hereinbelow, abbreviated as a font) which is displayed for every character is directly transferred to a memory called a video RAM (hereinbelow, abbreviated as a VRAM) and a bit pattern in the VRAM is displayed on a CRT through a CRT controller. Thus, the next character can be displayed only after the display of one character line has been completed. Therefore, in case of scrolling a screen upwardly, the scroll is performed while producing a blank line at the bottom portion of the screen and after scrolling is completed, a character string which should be displayed is indicated in the blank line.
However, in order to start the display of a character string after scrolling is completed as mentioned above, it takes a longer time to display characters of one line and the scrolling is stopped during this interval. Due to this, the scrolling is not performed smoothly in spite of the dot refresh type CRT. The following technology can be considered to solve such a problem and realize smooth scrolling.
Namely, the number of bits in the VRAM is set to be one line larger than the number of bits which can be actually displayed on the CRT; a character string which should be displayed next is preliminarily produced in the above-mentioned one extra line by use of the foregoing technology; this is supported by an intelligent CRT controller, thereby accomplishing smooth scrolling. However, the CRT controller for this purpose is complicated because of the scrolling function and the total cost of the apparatus is high.